This invention relates to a binder system comprising a phenolic resin component and an isocyanate component, wherein the phenolic resin component comprises (a) an alkoxy-modified phenolic resole resin and (b) an oxygen-rich polar, organic solvent.
A well-known and commercially successful method for preparing foundry cores and molds is the xe2x80x9cCold-Box-Processxe2x80x9d or the xe2x80x9cAshland-Processxe2x80x9d. According to this method, a two-component polyurethane binder system is used for the bonding of sand. The first component consists of a solution of a polyol, which contains at least two OH groups per molecule. The second component is a solution of an isocyanate having at least two NCO groups per molecule. The curing of the binder system takes place in the presence of a basic catalyst. Liquid bases can be added to the binder system before the molding stage, in order to bring the two components to reaction (U.S. Pat. No. 3,676,392). Another possibility, according to U.S. Pat. No. 3,409,579, is to pass gaseous tertiary amines through a shaped mixture of an aggregate and the binder.
In both these patents, phenolic resins are used as polyols, which are prepared through condensation of phenol with aldehydes, preferably formaldehyde, in the liquid phase, at temperatures of up to around 130xc2x0 C., in the presence of divalent metal catalysts. The manufacture of such phenolic resins is described in detail in U.S. Pat. No. 3,485,797. In addition to unsubstituted phenol, substituted phenols, especially o-cresol and p-nonyl phenol, can be used (for example, EP-A-183 782).
As additional reaction components, according to EP-B-0 177 871, aliphatic monoalcohols with one to eight carbon atoms can be used to prepare alkoxylated phenolic resins. According to this patent, the use of alkoxylated phenolic resins in the binder results in binders that have a higher thermal stability.
As solvents for the phenolic components, mixtures of high-boiling point polar solvents (for example, esters and ketones) and high boiling point aromatic hydrocarbons are typically used. The polyisocyanates, on the other hand, are preferably dissolved in high boiling point aromatic hydrocarbons. In European Patent application EP-A-0 771 599, formulations are described, which eliminate or reduce the amount of aromatic solvents, as a result of the use of fatty acid methyl esters. The fatty acid methyl esters are used either as stand-alone solvents or with the addition of polarity-raising solvents (phenolic-components), or, as the case may be, aromatic solvents (isocyanate components). Cores manufactured with this binder system are particularly easy to remove from the mold tooling.
In practice, however, binder systems formulated according to EP-A-0 771 599, display a serious disadvantage. They produce smoke during the casting process, so much that in many foundries, they are not practical to use.
In order to comply with the increasingly higher environmental standards and health and safety requirements, there has for many years been a growing interest in binder systems which contain no, or very little aromatic hydrocarbon solvent, but produce cores with adequate tensile properties.
This invention relates to a binder system comprising a phenolic resin component and an isocyanate component, wherein the phenolic resin component comprises (a) an alkoxy-modified phenolic resole resin and (b) an oxygen-rich polar, organic solvent. The invention also relates to foundry mixes prepared with an aggregate and the binder, a process for making cores and molds, and a process for casting metals.
The binder system has a little or no odor and the exhibits a low incidence of smoke during casting. The cores produced with the binder exhibit good flexural strength, particularly good immediate strength, and are easily released from the molding equipment.